TY - JOUR A1 - Kampe, Heike A1 - König-Rinke, Marie A1 - Petzoldt, Thomas A1 - Benndorf, Jürgen T1 - Direct effects of Daphnia-grazing, not infochemicals, mediate a shift towards large inedible colonies of the gelatinous green alga Sphaerocystis schroeteri N2 - The influence of Daphnia galeata x hyalina grazing and of infochemicals released by the daphnids on the colony size and growth rate of the colonial gelatinous green alga Sphaerocystis schroeteri (Chlorococcales) was investigated in laboratory batch experiments run for 96 h. High zooplankton grazing pressure was exerted by a final concentration of 100 daphnids L-1 in the Daphnia treatments. Infochemicals were obtained by filtration (0.2 µm) of water from D. galeata x hyalina cultures (200 ind. L-1 exposed for 24 h). This filtrate was added to the S. schroeteri cultures in two concentrations corresponding to 7 and 50 daphnids L-1, respectively. The growth rate of S. schroeteri was neither affected significantly by direct Daphnia grazing nor by the presence of Daphnia infochemicals in comparison to the control. However, the portion of inedible S. schroeteri colonies (diameter > 50 µm) increased under direct grazing pressure, whereas the Daphnia infochemicals did not influence the colony size significantly. We conclude that the shift in colony size by direct zooplankton grazing denotes an effective defence mechanism against size selective feeding for colonial gelatinous green algae. This effective defence in combination with unchanged growth rates of the larger colonies (under non-limiting nutrient and light conditions) falsifies the assumption of a trade-off between minimising grazing losses and maximising growth by optimising the colony size. Y1 - 2007 UR - http://www.sciencedirect.com/science/journal/00759511 U6 - https://doi.org/10.1016/j.limno.2007.01.001 SN - 0075-9511 ER - TY - JOUR A1 - Tittel, Joerg A1 - Wiehle, Ines A1 - Wannicke, Nicola A1 - Kampe, Heike A1 - Poerschmann, Juergen A1 - Meier, Jutta A1 - Kamjunke, Norbert T1 - Utilisation of terrestrial carbon by osmotrophic algae N2 - Terrestrial-derived dissolved organic carbon (DOC) contributes significantly to the energetic basis of many aquatic food webs. Although heterotrophic bacteria are generally considered to be the sole consumers of DOC, algae and cyanobacteria of various taxonomic groups are also capable of exploiting this resource. We tested the hypothesis that algae can utilise DOC in the presence of bacteria if organic resources are supplied in intervals by photolysis of recalcitrant DOC. In short-term uptake experiments, we changed irradiation in the range of minutes. As model substrates, polymers of radiolabelled coumaric acid (PCA) were used, which during photolysis are known to release aromatic compounds comparable to terrestrial-derived and refractory DOC. Three cultured freshwater algae readily assimilated PCA photoproducts equivalent to a biomass-specific uptake of 5-60% of the bacterial competitors present. Algal substrate acquisition did not depend on whether PCA was photolysed continuously or in intervals. However, the data show that photoproducts of terrestrial DOC can be a significant resource for osmotrophic algae. In long-term growth experiments, interval light was applied one hour per day. We allowed cultured Chlamydomonas to compete for ambient DOC of low concentration. We found higher abundances of Chlamydomonas when cultures were irradiated intermittently rather than continuously. These data suggest that photolysis of DOC supports algal heterotrophy, and potentially facilitates growth, when light fluctuations are large, as during the diurnal light cycle. We concluded that osmotrophic algae can efficiently convert terrestrial carbon into the biomass of larger organisms of aquatic food webs. Y1 - 2009 UR - http://www.springerlink.com/content/101191 U6 - https://doi.org/10.1007/s00027-008-8121-2 SN - 1015-1621 ER -